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    <p>I've checked Cassandra 1.1 code and noticed interesting behavior when writing with CL.ALL in multi DC deployment. <strong>Probably I've interpreted code wrong</strong>.... anyway: </p> <p>on the beginning they are collecting IP addresses of nodes to send row mutation - this is independent from consistency level provided by the client. In 1.0 it were all nodes from all DCs, from 1.1 they get all nodes from local DC plus one node from each remote DC (the remaining nodes are as "forward to" in the message). Each mutation will be send by separate thread, so the requests can run in parallel. Each such mutation is being handled as a message by messaging service. When node in remote DC receives message, it forwards it to remaining nodes, which are provided in "forward to".</p> <p>The consistency level provided by the client, defines number of nodes which must acknowledge received message. In case of CL.ALL this number is determined by replication factor - now is getting interesting: since we've send message to all nodes from local DC and to nodes from remote DCs, we will get also acknowledgement from those remove nodes too - yes this is still the number which is defined by replication factor, but depending on notwork latency, we can not be sure which nodes has conformed received message - could be mix from nodes from local and remote DC, but could be also only nodes from local DC. In the worst case, it could happen, that none of the local nodes got the message, and confirmation come from remote DCs (if you have many). This means - <strong>writing with CL.ALL does not grantee, that you can immediately read message from your local DC</strong>.</p>
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    1. POCassandra - write with CL.ALL with multiple data centers
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    1. USMaciej Miklas
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    1. COCL.ALL will block until literally *all* replicas respond. That means all nodes in the local datacenter *and* all nodes in remote datacenters. When writing with CL.ALL, when a write returns with a success status, you are guaranteed to see that write everywhere.
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      1. USnickmbailey
    2. COThe amount of expected responses from replicas is being calculated by WriteResponseHandler#determineBlockFor. WriteResponseHandler is an abstract class, created by factory method: AbstractReplicationStrategy#getWriteResponseHandler - in case of EACH_QUORUM factory creates: DatacenterSyncWriteResponseHandler, but for CL.ALL it returns: WriteResponseHandler, and it is not DC aware. It looks to me, that CL.ALL waits only for xx responses, where xx is the replication factor. Or maybe I am missing something....
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    3. CORight. But when using NetworkTopologyStrategy, getReplicationFactor() returns the *total* replication factor in all datacenters. It goes through each configured datacenter and adds the RF from that dc to the total and returns that. So ALL will block for a response from every replica in all datacenters.
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